Does the fan fit?
I wanna buy a Prolimatech Megahalems Rev B. Intel CPU Heatsink and on the site it also sell a fans clip for 38mm fans. I also see some 120mm x 25mm fan on the same page of the heatsink, but i don't know which one fit to that exact sizes. Is it possible to mount 2 fan on the Prolimatech Megahelems revB?
Go for the 25mm fan and make sure that there are no clearance issues between the fan and the RAM. If the RAM is too tall (with mini heatsinks) this must be checked out. The issue is usually with the 1st RAM slot (closest to the CPU). My Corsair Dominator RAM has a 1.0mm interference issue with the Prolimatech Megahalems Rev B cooler. If you run into interference issues, and have access to a milling machine, mill out a step on the fan housing to provide the necessary clearance.
As to your 2nd question - yes, you can install 2 fans in "push-pull" configuration, and the horizontal direction (not North-South) is preferable. Just my preference.
The cooler will work much better with 2 fans attached with clips.
Check the cooler base for flatness, and if required, do some polishing (sometimes referred to as lapping) to make the base flat. Increased flatness will reduce temps substantially. Lowered my idle temps 5 degs to 8 degs C by polishing my V8 cooler base.
Polishing is a process by which you use different progressively finer grits of silicon carbide (aka wet or dry) abrasive paper and polish away the surface of an object with the objective of making that surface more flat. In doing so, the finish is also improved to a finer finish.
This link will provide more information - http://en.wikipedia.org/wiki/Polishing
If you are not experienced in the polishing process, do not attempt it.